3.3.2: Internal Anatomy of the Primary Stem

Learning Objectives

• Identify the structures representing each of the three tissue systems in stems.
• Compare the structure of solenosteles, eusteles, and atactosteles.
• Explain the arrangement of leaf traces, leaf trace gaps, branch traces, and branch trace gaps.

The primary stem refers to the herbaceous (non-woody) stem, which has not undergone secondary growth (the growth that produces bark and wood). Some species (all monocots and some eudicots) remain herbaceous for their entire lives, maintaining the primary stem. Other species of eudicots initially form a primary stem but later become woody, replacing the primary stem with the secondary stem. The anatomy of the stem (internal structure) can be examined through longitudinal sections (cutting the stem lengthwise) or in cross sections (cutting a slice of the stem perpendicular to the length).

All three tissue types are represented in the primary stem. The epidermis is the dermal tissue that surrounds and protects the stem. The epidermis typically consists of one layer of cells. A waxy cuticle on the outside of these cells limits water loss. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. Pores in the epidermis called stomata (singular: stoma) allow for gas exchange. Each stoma is bordered by a pair of guard cells, which regulate stomatal opening. While stomata are present in stems, they occur at higher densities in leaves. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Trichomes extend from the epidermis of this Datrua inoxia stem. Image by Silk666 (CC-BY-SA).

Ground tissue fills much of the stem, forming the cortex directly within the epidermis and the pith (if present) in the center. The outermost portion of the cortex is usually a few layers of collenchyma cells. The remainder of the cortex and pith consist of parenchyma cells. The arrangement of vascular tissue in the stem depends on the species (see below).

Cross sections reveal three possible arrangements of vascular tissue (steles) in the stem. The first arrangement (solenostele) is present in a few eudicots, such as basswood (Tilia). In the solenostele, the vascular tissue appears as a continuous ring (vascular cylinder; Figure \(\PageIndex{2}\)). The interfascicular regions (pith rays) of parenchyma cells that separate vascular tissue are thus extremely narrow. The second arrangement (eustele) is present in most eudicots such as sunflower (Helianthus) and buttercup (Ranunculus). In the eustele, vascular tissue is clustered into distinct vascular bundles arranged in a ring, allowing for thicker interfascicular regions in between them (Figure \(\PageIndex{3-4}\)). In these solenosteles and eusteles, the vascular tissue separates ground tissue into an outer cortex and central pith. The third arrangement (atactostele) is present in most monocots, such as corn (Zea mays) and a few eudicots. In the atactostele, vascular bundles are scattered throughout the stem (Figure \(\PageIndex{3, 5}\)). While vascular bundles near the outside of the stem are packed more densely in this third arrangement, their distribution is somewhat disorderly. There is no distinction between the cortex and pith in the third arrangement.

Figure \(\PageIndex{2}\): Cross section of a common St. John's Wort (Hypericum perforatum) stem, illustrating a solenostele. The epidermis borders the entire root. The central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3–5 cells thick just inside the epidermis) are composed of parenchyma cells. Vascular tissue, forming the vascular cylinder, is composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith. Image by RolfDieterMueller (CC-BY)

Figure \(\PageIndex{3}\): In eusteles (left), vascular bundles are arranged around the periphery of the ground tissue. The xylem tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. Primary phloem fibers cap the vascular bundles. In atactosteles (right), vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue.

Figure \(\PageIndex{4}\): Cross section of a sunflower (Helianthus) stem, illustrating a eustele. Vascular bundles are arranged in a ring. The interfascicular regions between the vascular bundles are thick compared to the solenostele, where they were too thin to be visible. The ground tissue is separated into an outer cortex and a central pith. The epidermis borders the entire stem. Image labeled from Berkshire Community College Bioscience Image Library (public domain)

Figure \(\PageIndex{5}\): Cross section of a corn (Zea mays) stem, an example of an atactostele, at 40X magnification. Vascular bundles are scattered but are at a higher density near the outside of the stem. In contrast to solenosteles and eusteles, the ground tissue is not separated into a pith and cortex. The epidermis borders the entire stem. Image labeled from Berkshire Community College Bioscience Image Library (public domain)

The cells of embryonic tissue called the procambium (see Meristems) divides to produce primary xylem internally and primary phloem externally. In some vascular bundles, some procambial cells remain and form the fascicular cambium in the center of the vascular bundle. Once the stem has finished lengthening, sclerenchyma fibers called primary phloem fibers are produced just outside of the primary phloem. The primary phloem fibers of each vascular bundle are sometimes called phloem caps (bundle caps). If primary phloem fibers surrounded the entire vascular bundle, they form a bundle sheath (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): A vascular bundle in the stem of corn (Zea mays) at 400X magnification. The protoxylem vessel element matured prior to the metaxylem vessel elements of the xylem. At maturity, the protoxylem vessel breaks, leaving an air space. The sieve-tube elements and companion cells form the phloem. The bundle sheath consists of primary phloem fibers (sclerenchyma fibers) that surround the vascular bundle. Image labeled from Berkshire Community College Bioscience Image Library (public domain)

Vascular bundles connect leaves and stems. The strands of vascular tissue that connect the leaves to the stem are called leaf traces. Just above leaf traces are portions of stem without vascular tissue called leaf trace gaps. Similarly, branch traces connect axillary shoots to the main stem, leaving branch trace gaps just above them (Figure \(\PageIndex{7-8}\)).

Figure \(\PageIndex{7}\): The purple cylinder represents the vascular tissue in a stem. Vascular tissue leaves the stem to enter the leaf, forming a leaf trace. The space without vascular tissue just above the leaf trace is the leaf trace gap. The branch trace likewise produces a branch trace gap. Image by Melissa Ha (CC-BY).

Figure \(\PageIndex{8}\): A longitudinal section of a Coleus shoot tip. The leaf trace gaps are spaces without vascular tissue just above the leaf traces, which are strands of vascular tissue moving from the stem to the leaf. The three tissue system in this stem are represented by the epidermis (outer layer of dermal tissue, not labeled), pith and cortex (ground tissue), and vascular tissue between the pith and cortex. At the upper tip is the shoot apical meristem. Youong leaves (leaf primordia) emerge from either side. Axillary buds emerge just above where each leaf or leaf primordium meets the stem. Image by Melissa Ha (CC-BY).

Attributions

Curated and authored by Melissa Ha from the following sources:

• From 5.4: The Stem from Introduction to Botany by Alexey Shipunov (public domain)​
• From 30.2 Stems from Biology 2e by OpenStax (licensed CC-BY)